The invention relates generally to hydraulic systems and more particularly to an improved hydraulic system embodying a demand compensated principle in which a minimum feedback fluid pressure is maintained.
In the past, systems such as that shown in the U.S. Pat. No. 2,892,312 granted to J. R. Allen et al. maintained a constant pressure increment or standby pressure between a pump and a motor. This pressure increment was determined by internal biasing in the output control mechanism of the pump which opposed the pilot line fluid feedback. With this arrangement, a first portion of the pressure increment was required to compensate for pressure drops between the pump and the motor at the maximum flow rates and a second portion was required to satisfy any constant pressure requirements in the hydraulic circuit. When high, constant-operating-pressure motors were included in the hydraulic circuit, the second portion became substantial and was always imposed over and above the total pressure required to compensate for pressure drops and to supply the fluid motors. Any pressure increment above the maximum required pressure for the motors becomes a serious power drain and requires that the mechanical components be designed to withstand higher pressures than would be required by all the fluid motors at their maximum pressure demand.
Attempts at solving this problem have included such systems as those disclosed in U.S. Pat. No. 3,754,400 granted to D. J. Parquet and the U.S. Pat. No. 3,826,090 granted to J. M. Bahl. These systems provided pressure boosting means for satisfying the high, constant-operating-pressure equipment and a means for providing a fixed pressure ratio between the pressure required at the pump and at the motor which would compensate for the pressure drop therebetween. However, this fixed pressure ratio while being satisfactory for narrow operating pressure ranges was unsatisfactory for wide operating pressure ranges because a satisfactory compensation for pressure drop at a low pressure was magnified to an excessive compensation at a high pressure.